While it is a useful test and agree should probably be done, especially with an unknown engine, it is not the end all be all of tests. Especially when where are several other things to check first, things that are far more likely and easier to do. air leaks will also show up on the plugs or a bike that seems to be impossible to jet.
Eliminate the obvious variables before assuming the worse.
The fact he just bought and doesnt really know what was done and when mean he really should NOT be just changing stuff just to see what it will do, jets included.
Just regurgitating information you have read doesnt cut it.